This part of ISO 1833 specifies a method, using a mixture of formic acid and zinc chloride, to determine the mass percentage of viscose, certain types of cupro, modal or lyocell, after removal of nonfibrous matter, in textiles made of mixtures of viscose, certain types of cupro, modal or lyocell, with cotton, polypropylene, elastolefin and melamine. IMPORTANT If a cupro or modal or lyocell fibre is found to be present, a preliminary test is carried out to see whether it is soluble in the reagent. The method is not applicable to mixtures in which the cotton has suffered extensive chemical degradation, nor when the viscose, cupro, modal or lyocell fibre is rendered incompletely soluble by the presence of certain permanent finishes or reactive dyes that cannot be removed completely.